As average fuel prices steadily climb, there remains a need for ever more efficient internal combustion engines. With the goal of increasing engine efficiency, improvements to fuel injection technology have been made. Many engines today employ port fuel injection systems where port fuel injectors are disposed along an intake path that leads to the cylinders of the engine. Accordingly, the air and fuel are mixed before entering the cylinders of an engine that has port fuel injection.
In an effort to further increase engine efficiency, many manufactures are now switching from port fuel injection to direct fuel injection. In direct fuel injection systems, direct fuel injectors are positioned to directly inject fuel into the cylinders of the engine. Accordingly, the air and fuel are mixed in the cylinder itself. When direct fuel injection is employed, the fuel that is directly injected into the cylinders has a charge cooling effect. That is, the fuel being directly injected into the cylinders reduces the temperature of the intake charge as latent heat is absorbed from the intake charge to evaporate the fuel. This reduction in temperature allows for high compression ratios to be used, which in turn increases the efficiency of the engine. However, the efficiency increase associated with direct fuel injection is offset by poorer mixing of the air and fuel mixture relative to port fuel injection systems. Additionally, direct fuel injection increases the pumping mean effective pressure of the engine, which translates into less efficient engine throttling at low to midrange engine speeds.
To capitalize on the efficiency advantages of direct fuel injection while attempting to minimize some of the noted draw backs, manufacturers are now fitting engines with combined port fuel injection and direct fuel injection systems. While these combined fuel injection systems can achieve efficiency advantages over engines that have only port fuel injection or only direct fuel injection, the efficiency gains of these combined systems are limited by the parasitic friction losses associated with the need for simultaneously driving two fuel pumps for supplying both the port fuel injectors and direct fuel injectors. Such parasitic losses are particularly limiting with respect to the fuel pump that supplies fuel to the direct fuel injectors because a high fuel pressure is required for direct fuel injection. What is needed is a fuel injection system that combines port fuel injection with direct fuel injection and reduces the parasitic losses associated with the high-pressure fuel pump that supplies fuel to the direct fuel injectors.